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Note: This document was prepared informally and contains jargon and unconventional abbreviations.
Note: References to "figures" in this text are references to houndouts that were given on the trip. The photos here were taken on the field trip and are included for web site viewers.
We will attempt to see many representative formations and features of the Paleozoic Ouachita rocks in Arkansas and Oklahoma. Also, we will look at some of the formations in the coastal plains sediments that border the Ouachitas in the south and east, plus some features related to the intrusive rocks of Cretaceous age in Arkansas.
Lamprophyre dike (Tim Wilson is sampling it) cuts through the core of an anticline in the contact-metamorphosed Stanley Fm. The cross-cutting relations here and the hornfelsed nature of the Stanley provide physical evidence that the Magnet Cove magmatism post-dated the Ouachita orogeny. --Jones Mill Quarry
Click here to see additional photos from the trip.
Planned Route: Take Rt. 7 south to Dardanelle, then take Rt. 27 to Danville. At Danville, take Rt. 80 west to Waldron. Go through Waldron on Rt. 80 until it comes to Rt. 28. Go west on 28 to Heavener (the road becomes Rt 128 in OK). At Heavener go south on U.S. 270/59. The road cut will be on the left and right about a mile or so past Heavener.
Stop 1. AHeavener Road Cut@ on U.S. 59/270:
Upper Atoka Formation and Lower Hartshorne Formation
Stop 1 is located at NW3, SE3, NW3, Sec. 36, T. 5 N., R. 25 E. The lower part of this outcrop is dominated by shale. The Hartshorne formation begins with the first sandstone unit as you progress up section. The Hartshorne, then, is a sequence of sandstones and shales, plus near the top a thin coal bed, about two feet thick. These formations are of Pennsylvanian age. The Atoka defines the Atokan epoch. The Desmoinesian epoch starts with the Hartshorne formation.
The main emphasis of this outcrop is to look at very interesting features that occur in the Hartshorne formation here. In many places, such as in Russellville, the Hartshorne represents fluvial sediments. The very thick Hartshorne ledges that we see in many places are considered to be distributary channels on a deltaic system. However, many parts of the Hartshorne represent inter-distributary areas (swampy areas between distributary channels). This section is considered to represent this kind of sediment. Shale layers represent long periods of quiescence where only very fine sediments accumulate. The coaly layers would represent times when the plant growth became so much that most clay was kept out, and accumulations of plant debris made peat bogs. The intervening sandstones are considered to represent crevasse-splay deposits that probably represent periods of flooding. Crevasse-splays occur in this kind of environment when part of a levee breaks through during the flood stage. When it breaks through, water rushes through the levee, eroding it heavily, and washes sand and other sediment that was part of the levee system out into the interdistributary areas, filling in low spots with sand. Under this kind of circumstance, trees may die. We will look for fossilized tree stumps. Near the top of the road cut is a very large fossilized tree stump. In other places, small ones that would represent two or so inch wide trunks can be found. These occur mostly in the sands. In comparison with the Atoka rocks that we saw at Blue Mountain Lake dam, the Atoka here represents the stuff that was on the shelf, such as what is referred to as the section near Clarksville. After this outcrop, we will head south, and we will cross the Choctaw fault, which is a thrust that brought rocks from farther south up towards the north. It will be as if we went quite a bit further south after we cross that fault.
Continue on to Stop 2: Continue south of U.S. 59/270. A little more than two miles the road enter Hodgen. Take note of odometer. We will go 8.2 miles south of Hodgen. Just after crossing Shawnee Creek will be a road going to the west to the Ouachita Vocational Technical camp. Park there for stop 2.
Stop 2. John's Valley Shale at Stap, OK on U.S. 59/270
The location of stop 2 is in the center of the E2 of Sec. 12, T. 3 N., R. 25 E. Shortly after we pass through Hodgen, we crossed the Choctaw fault. The rocks have actually been transported some distance north from their original site of deposition by the Choctaw and other faults of the Ouachita orogenic system. The John=s Valley shale is of Lower Pennsylvanian age ("Morrowan", of similar age as the Bloyd shale). Thus the John=s Valley shale underlies the Atoka formation (see Figure 1). The John=s Valley shale, as its name implies, is a unit that is dominated by shale, with some thin beds of sandstone as well. What characterizes this particular unit is a very unusual feature. At various horizons within the John=s Valley shale, boulders or boulder beds occur with a very wide assortment of rock types, represented by boulders and cobbles. Many of these rock types are limestones and dolomites that are similar to the shallow water deposits that we saw in northern Arkansas. Similar rocks of lower to middle Paleozoic age occur in Oklahoma as well. Many debates have arisen to explain these. We will attempt to explain the nature of this deposit later in class.
Toward the south end of the outcrop, more of the normal lithologies that are common in the John"s Valley shale can be seen. Among these are numerous occurrences of graded bedded sandstone/siltstone/shale. This group will be recognized by a systematic distribution of finer and finer-scale cleavage within single beds of sandstone/siltstone. Graded bedding like this is suggestive of deep-water sedimentation by turbidity flows. We will have to put this observation together with the boulder beds and the limestone clasts in an interpretation of John=s Valley shale. From here on out in the Ouachitas, we are going to be seeing features suggestive of deep-water sedimentation.
Continue on to Stop 3: Continue south on U.S. 59/270 for 2.4 miles. Turn right (south) on U.S. 259. Go 0.6 miles and stop. Park on the right side of road and walk back to outcrop.
Stop 3. Atoka Formation on U.S. 259 South of U.S. 59/270
The location of this outcrop is SE3, NW3, SE3, Sec. 17, T. 3 N., R. 26 E. in Le Flore County, Oklahoma. This outcrop in the Atoka formation is stratigraphically above the outcrop of John=s Valley shale. Again, because of the northward-directed thrust faulting, these rocks are actually well north of where they were originally deposited, and represent facies that are well south of what was seen back at Stop 1. The Atoka here is very characteristic of the majority of Atoka rocks in the frontal Ouachitas. These strata are dominated by shales and mudstones. However, there are also sandy beds. The sandy beds commonly have bottom marks which we will look for, and graded bedding. It is said that complete and incomplete Bouma sequences can be found. The kinds of layers that occur here are characteristic of a submarine fan environment. Thus the Atoka formation exposed here is considered to represent sediments similar to the Atoka that we saw at Blue Mountain Lake dam, except that here we are on a part of a fan that did not have a channel.
Continue on to Stop 4: Continue on U.S. 259 south for 3.6 miles to the intersection with state highway 1 (Talimena Drive). Turn right onto the Aon@ ramp for Talimena Drive. Then go east on Talimena Drive. Go about 3.7 miles to just north of a hairpin curve on Talimena Drive to a large Road cut.
Stop 4. Upper Part of Stanley Group Plus Lower Part of Jackfork Sandstone
This outcrop is located at NW3, SE3, SW3, Sec. 36, T. 3 N., R. 25 E. Le Flore County, OK. There is some debate as to where the best position to place the contact between the Stanley Group rocks and the Jackfork Sandstone. Traditionally, the contact has been put at the top of a very thin, but regionally-persistent siliceous shale called the Chickasaw Creek siliceous shale. However, the rocks above this siliceous shale for some distance up the outcrop are not significantly different from those beneath the siliceous shale. Therefore others prefer to put the contact at the base of the first thick massively-bedded sandstone that occurs way up at the top of this Road cut.
The Stanley Group is characterized by dark shale, but with sandstone interbeds of varying thicknesses. The rocks here are quite typical of the Stanley. If we work our way up section, we should be able to find some pretty good examples of graded beds. I collected a sample here which contained units A, B, C, and D of a complete Bouma cycle. That sample came from well up the hill. Though it is a bit of a rough climb, it will be worthwhile to get a sample of the Jackfork sandstone at the top of this hill.
Again, the sediments here are indicative of deep-water sedimentation by turbidity flows, probably within the deep basin off the southern coast of North America of that time. The age of the Stanley Group corresponds with rocks such as the Fayetteville Shale and Pitkin Limestone that we saw in northern Arkansas. The Stanley sediments are surely indicative of much deeper water sedimentation (see Figure 1). The Jackfork Sandstone marks the beginning of the Pennsylvanian. That time period represents a continent-wide low sea level stand. It is thought by some that exposure of more rocks on the craton led to the rapid influx of coarser sediments down into the base of deep channels and canyons off the shelf. The source of Jackfork sediment, however, is not thought to be from the north, but rather from the east from the rising Appalachian Mountains. This is because abundant bottom marks in the Jackfork sediments (not seen here) suggest derivation from the east, that is, westward-directed turbidity flows.
Continue on to Stop 5: Continue on Route 1 going east. Cross into Arkansas and follow this all the way to Mena (the road becomes Route 88 in Arkansas). At Mena, catch U.S. 71 south. We will go about 20 miles south on U.S. 71 to the vicinity of Hatton. We will turn left (east) on a county road where you just enter town. No more than about a half mile down that road on the right should be the entrance to the Meridian Company Hatton Quarry (formerly Hertzhog Inc.).
Stop 5.
Hatton Quarry
Arkansas Novaculite and Hatton Tuff of the Lower Part of the
Stanley Group
This location falls on the Wickes quadrangle (see Figure 3) and the quarry occupies part of the NE3, Sec. 1, T. 5 S., R. 32 W. The other part is in the NW3, Sec. 6, T. 5 S., R. 31 W. The quarry here has been active for a number of years. For a long time they produced construction aggregate from the Arkansas Novaculite, and the more recent years they began to develop the Hatton Tuff. Weathered Hatton Tuff does not make good road aggregate, but this fresh tuff, where weathered rock has been stripped off, makes some of the best construction aggregate that comes out of Arkansas. The Hatton Tuff was described in a classic paper on Ouachita geology by Meiser and Purdue (1929), U.S.G.S. Bulletin 808. This unit is fairly extensive in the southwest portion of the Ouachitas, and has recently been recognized even in the Hot Springs area. Meiser and other geologists have made very strong documentation that this rock is indeed a pyroclastic tuff. However, when this quarry began to produce from this layer, some geologists thought that perhaps this rock was not actually part of the tuff that had been mapped by Meiser, but was rather a shallow intrusive dike, probably related to the Cretaceous magmatic rocks near Magnet Cove and Little Rock. It is a significant question as to whether this is an intrusive rock, or part of the Hatton Tuff. If this is an intrusive dike, then it will be very difficult to locate other such dikes like it since it would be here more-or-less by happenstance. However, if it is the Hatton Tuff, this unit has been mapped and other occurrences of this high quality rock can easily be found by referring to Meiser=s map (see Figure 4). We will look for features in this rock to see if it is more characteristic of a tuff or of a shallow, intrusive dike.
The Stanley Group in which this tuff layer resides, is of Mississippian age. This tuff and a few others like it, also in the lower part of the Stanley, are the only igneous rocks of this age in the Ouachitas. However, some other volcanic layers similar to these occur in the subsurface Ouachita extension southwestward in Texas. No intrusive rocks of this age occur in the exposed Ouachitas, and the source of this tuff layer is interpreted to have been to the south. We will discuss the significance of this volcanic rock later in class.
We will also try to look at the Arkansas Novaculite here. Novaculite is actually a chert, but a very special kind of chert that occurs only here in Arkansas, and also in similar rocks in the Marathon fold belt of southwest Texas (a belt of deformed rocks that are considered to be part of the Ouachita orogenic belt, most of which is covered by coastal plain sediments. Chert and novaculite both are microcrystalline quartz, but in microscope the grain size of the novaculite is distinctly larger, and is not typical of cherts. It is thought that the very low grade of metamorphism that has affected much of the Ouachitas, is responsible for recrystallization of the microcrystalline quartz in the novaculite, turning it into the rock that it is. The Arkansas novaculite and other cherts in the Ouachitas are considered to have accumulated in a deep sea basin. The novaculite stratigraphically underlies the Stanley Group, and is Devonian to lower-most Mississippian in age. It is an age equivalent to such rocks seen in northern Arkansas as the Boone formation and Chattanooga shale.
For over a hundred years, Arkansas Novaculite has been considered perhaps the best source of abrasives for specialty sharpening stones for not only pocket knives, but dental instruments and other industrial applications. It is marketed the world over as "Arkansas stone". However, recently some manufacturers have produced synthetic ceramic sharpening stones, and have even called them Arkansas stone, confusing the issue and creating problems for the producers in Arkansas.
Continue on to Stop 6: There are two possible alternate routes to the next stop. I will only include here alternate 1 (alternate 2 is letting Danny Smith lead us through the wild back roads of Polk County to the next stop). We turn to U.S. 71 and go north to Mena. At Mena, catch Route 8 going east. After about 17 miles we will come to the town of Bigfork. About a mile south of town, you will cross a creek called Bigfork, and go through the Alpine Spring and Abernathy Spring community. About 22 or 3 miles further east on Route 8, come to a county road that goes south toward Little Missouri Falls Recreation Area. Go south nearly 2 miles to the vicinity of Mosquito Gap.
Stop 6. Slates of the Missouri Mountain Formation and the Stanley Group
This stop is located on the Bigfork, Arkansas 72 minute quadrangle. We will look at two outcrops. One is located in SE3, SE3, Sec. 33, T. 3 S., R. 27 W., the other in NW3, SE3, Sec. 4, T. 4 S., R. 27 W. The first one, labeled Stop 6A on Figure 5, is somewhere off to the north of the main road on a very old road not shown on the map that goes off to the north at the same place where the dirt road that goes on to Slatyington Mountain on the south comes in. The other, labeled stop 6B, is where a shale pit is marked on the map.
This area is approaching the central part of the Benton uplift. The Benton uplift is a large anticlinorium that exposes rocks that were among the mmost deeply buried within the Ouachita origin. The rocks here are of a low metamorphic grade, shales having been turned into slates. Slates used to be quarried in this area in years past (for hand-held slate tablets for school children, I am told). The town of Slatyington no longer exists. Stop 6A is in the Missouri Mountain shale, shown as the darker color on the Meiser and Purdue (1929) map reproduced as Figure 6. The slates in the Missouri Mountain shale are reds and greens in color. The other quarry is in the Stanley Group, and the slate there is black. This should not surprise you, having just seen the Stanley in earlier stops, and seeing the black shale there. Break some of these rocks with your hammer and notice the difference between the slate here and the Stanley shale farther to the west in Oklahoma. This should give you a good feel for the difference between shale and slate.
The lower Paleozoic rocks in the Ouachitas (see Figure 2) are dominated by fine-grained sediments. Cherts are also present. Cherts and abundant shale are considered characteristic of deep-sea type sedimentation.
Continue on to Campground. Return to Route 8 and turn east. At Norman, continue Route 8 which now joins Route 27 turning southeast (right). About 5 or 6 miles down the road, near the community of Caddo Gap, if time allows, we will stop and look at a highly contorted Arkansas novaculite and other rocks in the Road cut at Caddo Gap. Continue on Route 8 to Glenwood. At Glenwood turn west on U.S. 70. About 9 miles from there come to Kirby. At Kirby, U.S. 70 goes to the right. However, we continue straight and the road becomes Route 27 heading for Murfreesboro. At Murfreesboro turn right on Highway 19 heading for Lake Greeson and the Narrows Dam. Proceed to the Corps of Engineers area and follow road to the Narrows Dam. Turn left just behind the dam and cross the river. Follow signs to Parker Creek. The road gets to be a gravel road that is well marked most of the way. Follow sign to Parker Creek Campground.
Stop 7.
Cinnabar Mine in Jackfork Sandstone at Corps of Engineers
Recreation Area, Lake Greeson
If time allows before dark, or first thing in the morning, take a side trip to the Gap Ridge cinnabar mine. To get there, go to the end of the camping area where the road loops around (there is a loop going to the left and the road also forks off to the right there). As you begin the loop, there is an old trail going off into the woods with a chain across it. Park there. Walk about 100 yds up this trail then go up the hill bearing a little to the left. At the top of the hill are some old shafts and one addit.
The cinnabar was mined from the Gap Ridge Sandstone Member of the Stanley Formation here and at many sites along strike for a number of miles. In some places to the east it was mined in the Jackfork Sandstone. The Gap Ridge Member is in the uppermost part of the Stanley. We found cinnabar here in the past in some thin sandstone beds in an interbedded sandstone/shale interval inside a pit. Cinnabar occurs in some thin veinlets along with a white soft mineral (dickite clay?).
DAY 2
The field trip stops of Day 2 will focus on some post-Paleozoic features in Arkansas. The Ouachita orogeny is considered to have occurred in the late Pennsylvanian. The next youngest rocks that occur in Arkansas at the surface are Cretaceous sediments and Cretaceous-aged alkalic intrusive complexes. We will see the evidence of the unconformity between these rocks and the Paleozoic rocks, evidence for depositional environments following the Paleozoic, and evidence for the post-Paleozoic age for the intrusive complexes.
Directions to Stop 8: Leave the Lake Greeson camping area via Route 19 south to Murfreesboro. At Murfreesboro, turn right (southwest) on Route 27 heading south. Go about 5 miles to Route 26 and turn right (west). Drive 1.6 miles on Route 26 to the entrance of the quarry on the right. Enter the quarry and check in at the mine office. The best exposures are in the inactive part of the quarry on the west wall of the old quarry.
Stop 8. The DeQueen Formation (L.K., Trinity Group) at the Highland Quarry
This stop is discussed in detail in the enclosed article by J.D. McFarland of the Arkansas Geological Commission. Note here the contrast in the nature of these rocks with those just a little farther north in the Ouachitas. The rocks here are much less lithified than the Ouachita rocks, and also these lie horizontally. Drilling for water wells in this area would, at a fairly shallow depth, intersect hard Paleozoic rocks beneath these in the subsurface. There is an angular unconformity between these (see geologic map). This unconformity provides relative dating evidence that the Ouachita orogeny occurred sometime postdating the early Pennsylvanian, and predating at least the Cretaceous.
The DeQueen Formation consists of limestones, gypsum, mudstones, and minor sands. This mixture of carbonate/evaporite plus terrigenous siliciclastic sediments is interpreted to have been deposited in a broad, shallow lagoon just off the Cretaceous coast. This lagoon was protected by a reef called the Glenrose Reef. This reef is interpreted from rocks of similar age that occur in the subsurface farther south. The mudstones, siltstones, and sandstones have brackish water fossils in them. Toward the top, some of the rocks have calcite pseudomorphs after halite. The gypsum beds in the DeQueen Formation are discontinuous when looked at over large areas. The layers have an overall lenticular geometry. These observations suggest that the gypsum was deposited in depressions of limited areal extent on tidal mud flats. The calcite pseudomorphs after halite indicate that there were time periods of supratidal (above sea level) exposure with drying out of saline waters. Both the halite pseudomorphs and the gypsum probably indicate supratidal brine pools. Note: some zones near the middle of the highwall contain celestite nodules.
Continue to Stop 9: Return via Route 26 to Route 27. Take 27 north to Murfreesboro and just on the other side of town take Route 26 east through Antoine and on to Arkadelphia. At Arkadelphia catch I-30 going east to the vicinity of Malvern. Exit at U.S. 270 and go west. Immediately after that you will come to Route 51 on the right. Take 51 north to Magnet Cove. After passing Magnet Cove school, go another 22 miles (approximately) to where the road crosses Cove Creek. Park vehicles on the right.
Stop 9.
Magnet Cove Alkalic Intrusive Complex
Carbonatite at Cove Creek/Highway 51
This stop is labeled on the map given as Figure 7. The outcrop is actually just barely off the west edge of the map. It occurs in the northern part of Sec. 19, T. 3 S., R. 17 W. A summary of the Magnet Cove intrusive complex is provided by the Arkansas Geological Commission. This stop is labeled as B on the map included with that handout. The Magnet Cove intrusion is a world-class alkaline complex, including the most unusual of all igneous rocks, carbonatite. On the north side of the road and on the west side of the creek, some white rock is exposed. This is composed mostly of calcite. There are also some accessory minerals in with the calcite. Across the road is some nepheline syenite pegmatite.
Continue to Stop 10: Continue west on 51 a few tenths of a mile to an intersection with the road going off to the left. Take this road to the left (south). This road will, after about a mile, run into U.S. 270. Turn left (east) on 270. Go about 1.4 miles to where a house is on the left with a mailbox that says AParsley@. Pull onto the dirt trail that runs north into the woods and park.
Stop 10. Magnet Cove Intrusive Complex C Diamond Joe Quarry
Walk in on the trail through the woods to where the old Diamond Joe quarry is. This is on private land owned by an avid supporter of earth science and mineralogy. Permission for entry should be obtained from Henry deLinde of Mablevale, AR. We are asked not to take any samples off of the exposed bedrock, however, we can collect samples from pieces of rubble that occur all over the quarry floor.
As we face the quarry, we are standing on Stanley Group shale. Over towards the right, the contact between the Stanley Group and the intrusive rocks is exposed. We will examine this contact (again, especially here, no sample taking). After that we will look at the nepheline-bearing syenite that makes up the main body of the quarry. Some of these rocks contain small, dark knots of black melanite garnet (these are titanium-rich garnets). We may also be able to find a number of xenoliths. If we go uphill around the right-hand side of the quarry (east side), we may be able to find a large boulder of Stanley formation. This rock is thought to be a large xenolith of Stanley within the intrusive rocks. Since this is not seen as an outcrop but as a large boulder, its xenolithic origin is questionable. If this is a Stanley xenolith, it provides a clear indication of the pluton being post-Mississippian in age. The AGC report also says we can find a contact between the "garnet pseudoleucite syenite" that makes up most of the quarry and "nepheline syenite pegmatite" at the top of the highwall to the northeast.
Continue on to Stop 11: Return to vehicles and go east on 270 about another mile to the entrance of Mid-State Construction Materials quarry. Check in at quarry office.
Stop 11. Stanley Formation and Arkansas Novaculite at Mid-State Construction Materials Jones Mill Quarry
Evidence for contact metamorphism from the Magnet Cove intrusive complex
The main objective of this stop is to see intrusive contact relationships between the Magnet Cove pluton and the Paleozoic age country rock. We will look at Stanley Group Ashales@ and "sandstones" that have been baked by contact metamorphism into hard, brittle hornfels. This demonstrates the intrusive nature of the adjacent igneous complex, which is just over the hill from this quarry. Compare these rocks to what you saw of the Stanley in other places.
In addition to the hornfelsed nature of the Stanley Group rocks here, we will also see a number of lamprophyre dikes and sills. Lamprophyre is a dark, mafic igneous rock that has ferromagnesian minerals as phenocrysts. These lamprophyres are evidently related to the alkaline intrusive complex. Since they cut across the Stanley Group rocks, this is additional field evidence for the post-Mississippian intrusion of this igneous complex. Actually, depending on where quarrying is at at any one time, dikes can be found cutting across folds as well. Thus the intrusion also post-dates folding. Since we have seen Pennsylvanian rocks (Atoka and Hartshorne) folded in the Ouachitas, the intrusion post-dates the Pennsylvanian based on field relationships we can see.
In an older part of the quarry, we can see Arkanas Novaculite that is intensely folded. The novaculite is no longer being quarried here.
Continue on to Stop 12: Go east on 270 and cross I-30 and head into Malvern. In the middle of town U.S. 270 takes a left turn, but you continue straight onto Rt 9. Go about 0.7 miles past that intersection. You will pass Pratt School on the right and then a small creek. Take the first left turn after that small creek. After taking that left, you will immediately take a fork to the right. Then a little over 0.2 miles further, take another fork to the right. This road will soon take a 901 dogleg to the left (east). This road is Sulphur Springs Road. From the dogleg, go about 0.9 miles to a dirt road that goes off to the right (south). It is just after a left bend in Sulphur Springs Road. Turn right and go a little over 0.4 miles on this gravel road to the second of two closely-spaced jeep trails going off to the right. Park there and walk 200-300 feet or so into the woods. You will see a pit dug out off through the woods on your left. Proceed to examine the steep highwall of the quarry pit.
Stop 12. Tertiary-Aged Wilcox Group Sediments Near Malvern
The Wilcox group sediments are of the oldest epoch of the Tertiary (Paleocene). Some of the features that we will look for that indicate a fluvial environment (perhaps delta plain) include the following: (1) a finding of upward sequence from sand up to clays (2) lignite layers. Lignite is a low-rank coal that did not undergo as deep a burial as the bituminous-type coals seen in the Pennsylvanian age rocks of Arkansas. These sediments indicate a different kind of paleo-environment as compared to the Cretaceous sediments seen at the first stop of this day (Stop 8).
Continue to Stop 13: Retrace route back to Malvern and I-30. Go east on I-30 to the vicinity of Benton. Continue on interstate to the Bryant exit (Hwy 183). Go south on 183 through Bryant, a total of about 5 miles, to the vicinity of Bauxite. There, Rt 183 will bend sharply to the right (west). After bending west, go less than one mile to where railroad tracks are on a bridge over the road. Immediately after that, turn left (you will pass some large buildings related to the old bauxite mining works here on your left). After about 0.6 of a mile, this road will join into another road coming in from the left. You will stay to the right here and join that road. After about 0.3 of a mile the road will fork and you stay to the left. About a 0.1 of a mile after the fork there is an intersection that we go through. About 2 mile further will be a trail going off to the right to the location of the next site.
Stop 13. Xenolith in Nepheline Syenite Pluton Near Bauxite, Ark.
This location was donated to the Arkansas Geological Commission by Alcoa in 1990 for the sake of scientific education. The Geological Commission does not allow any kind of sampling from this outcrop without written permission.
The xenolith is of some form of layered rock, giving it a distinctly sedimentary appearance. The very angular shape of the xenolith suggests it had broken off in a brittle fashion from the wall rock or roof rock while the pluton was intruding. This xenolith, however, is very unusual in that the minerals in these sedimentary-looking layers are largely sodic pyroxene. It is thought that chemical reaction between the alkalic magma and this xenolith converted the original clay and silt of the sedimentary rock into pyroxenes. This is a process called metasomatism. The xenolith is thought to be composed of one of the Paleozoic rock units. The xenolith would indicate that the intrusion of the syenite came after the Paleozoic. Also in this outcrop, numerous dikes of various sizes can be found. These represent late-stage magmatic processes.
The property on which this outcrop is located is part of an area of old bauxite mines. We will discuss in class later the processes which may have worked to produce the bauxite in this area.
Continue on to Stop 14: Retrace route back to I-30. Then take I-30 east toward Little Rock. At I-430 take I-430 north. Go a little past the Stage Coach Road exit to where an outcrop occurs on the right with another larger outcrop across the interstate on the left. Park your car there.
Stop 14. Big
Fork Chert, Arkansas Novaculite and a Cretaceous Dike,
I-430 near Stage Coach Road
The rocks in this outcrop are intensely deformed, giving an excellent view of the style of deformation of the Ouachitas. We will examine the outcrop on the west side of I-430. The north end of that outcrop has Big Fork chert that is overturned. Bedding dips to the north. Stratigraphic up, however, is towards the south. Structurally underlying the Big Fork (but stratigraphically Aoverlying@) is the black shale and thin chert beds of the Polk Creek Shale. A lot of shearing occurs along the contact between the Big Fork Chert and the Polk Creek Shale. Further towards the south in this outcrop the Polk Creek is in contact with the ArkansasNovaculite. Since the novaculite is younger than the Polk Creek, and yet underlies it structurally, this demonstrates that the beds indeed are overturned. Progressing further towards the south along the outcrop, the very massive bedded lower part of the novaculite makes transition into the interbedded chert and shale of the middle division of the Arkansas Novaculite.
All these rocks are intensely sheared and folded. Note here that the structural vergence is towards the south. This is an anomaly in the Ouachitas that is difficult to explain when thrusting is considered to be from the south toward the north (which should produce northward vergence). Another feature to look for in this outcrop is a dike of igneous rock that cuts through nearly vertically in this outcrop. This igneous rock has been weathered heavily and turned to clay. If lumps of the clay are dug up, sometimes a faint color difference within that clay gives one the impression of feldspar phenocrysts in a aphanitic matrix. This is a pseudomorphic replacement phenomena where the porphyritic original texture of the rock is preserved even though the rock has been totally turned into clay by weathering. On the very top of the outcrop is an area where a gravel deposit occurs. This is thought to be part of the Tertiary-aged deposits. So this outcrop combines exposured of some of the oldest rocks in the orogen, intense deformation of the Ouashita orogeny, Cretaceous intrusives, and Tertiary sediments.
Route back to Russellville: Continue north on I-430, cross the Arkansas River, and take I-40 west. Go about 70 miles to Russellville.
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